1. Field of the Invention
The invention relates to the field of production printing systems and, in particular, to reducing the amount of ink applied to a tangible medium during an ink jet printing process.
2. Statement of the Problem
A production printing system is a high-speed printer used for volume printing, such as 100 pages per minute or more. Production printing systems generally include a localized print controller that controls the overall operation of the printing system including, for example, host interfacing, interpretation or rendering, and lower level process control or interface features of print engines of the printing system. Host interaction may include appropriate adapters for coupling the printing system to one or more host systems that transmit print jobs to the printing system. The print jobs are generally encoded in the form of a page description language such as PostScript (PS), PCL, IPDS, etc.
In whatever form the print job may be encoded or formatted, the print controller within the printing system interprets the received information to generate sheetside bitmaps of the print job. The sheetside bitmaps represent the image to be printed on a side of a sheet of paper. Each sheetside bitmap generally comprises a 2-dimensional array of picture elements (“pixels”) that represent a corresponding formatted sheet of the print job. Each pixel may represent an encoded color value in accordance with the requirements of the particular print job encoding and the capabilities of the printing system on which the print job is to be printed and may be represented by one or more data bits.
The print controller stores or buffers the sheetside bitmaps in accordance with storage capabilities of the particular architecture of a particular print controller. The print controller then forwards the sheetside bitmaps to one or more print engines (sometimes also referred to as an “imaging engine” or as a “marking engine”). The print engines include one or more printhead controllers that control one or more printhead arrays. Each printhead controller is associated with a printhead array so that the printhead controller is the system controlling how the printhead array discharges ink onto a medium.
Presently, when the print controller receives a print job and generates sheetside bitmaps, each pixel in the sheetside bitmaps is represented by one or more bits per pixel. Pixels of 8 bits or more can be used to represent grayscale or index color. Thus, in many production printing systems, the print controller sends full 8-bit sheetside bitmaps to each of the printhead controllers in the print engine. A full sheetside bitmap includes an entire set of pixels for an image.
After receiving an 8-bit sheetside bitmap, the printhead controller performs a halftoning or screening process on the 8-bit sheetside bitmap, which generates a 2-bit sheetside bitmap of dots varying in size and/or spacing. The printhead controller then processes a bitmap mask assigned to that controller to determine which pixels in the 2-bit sheetside bitmap to print through its associated printhead array. For example, a printhead controller may determine that its associated printhead array is to print every other line of the 2-bit sheetside bitmap, or every other pixel of the 2-bit sheetside bitmap. When printed, the dots appear to the human eye as continuous tone imagery.
In color printing, the halftone process is repeated for each subtractive color, most commonly using the Cyan Magenta Yellow Black (CMYK) color model. The semi-opaque property of ink allows halftone dots of different colors to appear as full color imagery. Problems, however, can occur when certain colors in the halftoning process are produced. For example, a pixel may be represented by multiple color components of the CMYK color model using various size ink drops and positions. Certain colors are represented with multiple drops which can saturate certain printable mediums, such as paper, and distort and/or warp the medium. Thicker paper, in many cases, can absorb more ink than thinner papers but it is generally more expensive. Moreover, in large scale printing systems, such paper is fed to the printing system in large rolls. Thicker paper generally means less printable paper per roll and thus more roll changes during printing operations. Additionally, ink density (i.e., color quality of the ink) can be lost as ink soaks into the paper rather than remaining on the surface. Further still, thicker paper is generally heavier resulting in higher shipping costs (e.g., when printed upon and shipped as part of a customer order).
To reduce ink penetration during halftone printing, color ink jet printers may limit the amount of ink deposited on a pixel using an arbitrary threshold that is believed to decrease ink penetration. That is, the ink being applied to the paper is simply limited to an amount that is believed to not penetrate the paper. For CMYK printing, this value is likely to be in the range of 150-300%, where 400% represents each CMYK component present at maximum value. Generally, displays, scanners, and cameras have larger color gamuts than printers. Limiting the ink in this manner further restricts the color gamut of the printer. In other words, the previous color ink limitation reduces the quality of a printed image even more than the already limited color gamut of the printer. A solution is therefore needed that reduces or prevents ink penetration when printing via ink jet printers while providing a larger perceived printer color gamut.